Alan P. Berens

Statistical Evaluation of NDE Reliability in the Aerospace Industry

The goal of this paper is to review the statistical methods used in the aerospace industries to evaluate NDE reliability. The techniques presented are consistent with the damage tolerant design and structural maintenance philosophies of the aerospace industry. The first part of this paper establishes the evaluation criteria and discusses the history of NDE reliability evaluations. The second part describes the state-of-the-art analysis methods through examples from the retirement for cause (RFC) inspection system evaluation. The last part of the paper discusses some techniques used to rate operator performance and deal with false calls.

Estimating the Statistical Properties of Crack Growth for Small Cracks

Abstract A series of statistical studies were conducted on two sets of fatigue crack growth data which were generated under variable amplitude loading from cracks that initiated in 7475-T7351 Aluminum down in the bore of typical aircraft quality prepared holes. The study considered fatigue crack growth (life) behavior and fatigue crack growth rate behavior. Generally, the study showed that small crack growth behavior was similar to that observed for larger cracks and that a stress intensity factor parameter could be used to describe the mean trend of the fatigue crack growth rate behavior via a power law relationship.

Estimating the Distribution of the Sizes of Flaws Remaining After an Inspection

Abstract : The U.S. Air Force plans for maintenance and retirement of aircraft based in part on fatigue crack growth models. Periodic inspections are used to help assess airworthiness and plan for future inspections. Nondestructive inspections are not perfect so some cracks are missed and the likelihood that an individual crack is detected is a function of the size of the crack when inspected. Additionally, the crack size distribution is related to the number of flight hours the aircraft has experienced, so not all inspection results come from the same distribution. In a recent study several models were compared that utilize the capability of the inspection system and the variation between aircraft and times of inspections to estimate the distribution of sizes of cracks that were missed during the inspection. This white paper summarizes those results and identifies some methods for extending them.

Index Size Determination with Varying Coil Diameters for Improved Automated Eddy Current Inspections on Aerospace Engine Components

The effect of index size and coil size on eddy‐current inspection throughput rate, and resulting Probability of Detection (POD), are being examined using the USAF’s Eddy Current Inspection System (ECIS). Original estimates of potential maximum index sizes were made by applying a sampling and shifting algorithm to the experimental data. Better estimates of the new maximum index sizes were obtained by fitting Gaussian functions to the experimentally determined line‐spread functions.